Re: [PATCH RFC 0/4] slab, rcu: move and consolidate TINY_RCU kvfree_rcu() to SLAB

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Jan 23, 2025 at 11:37:17AM +0100, Vlastimil Babka wrote:
> Following the move of the TREE_RCU batching kvfree_rcu() implementation
> to slab, we still have the simple non-batching implementation in tiny
> RCU, and RCU implementation specific ifdefs in slab code.
> 
> Finish the move and integration into slab. Allow using the simple
> call_rcu() based implementation also with tree RCU when SLUB_TINY is
> enabled, as its goal is also to limit memory footprint with less concern
> for top performance.
> 
> In order to avoid RCU having to recognize the fake callback function
> pointers (__is_kvfree_rcu_offset()) when handling call_rcu(), implement
> a callback that can calculate the object's address from the embedded
> rcu_head pointer without knowing the specific offset (previously SLOB
> would not have made it possible, but it's gone now).
> 
> After this series, AFAIK only the following kvfree_rcu specific code
> remains in RCU:
> 
> - a call to kfree_rcu_scheduler_running() from rcu_set_runtime_mode()
> 
>   - probably necessary and a generic registration interface would be
>     unnecessary bloat?
> 
> - declarations of kfree_rcu() API in include/linux/rcupdate.h
> 
>   - could be moved to slab.h after checking for/fixing up potential
>     missing includes
> 
> git tree:
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=b4/slub-tiny-kfree_rcu

Looks good to me, except typo in third patch 'kmalloc of vmalloc allocation'
should be 'kmalloc or vmalloc allocation'.

Maybe also add some comments on top of 'kvfree_rcu_cb()'. Like:

/*
 * Given an rcu_head embedded within an object at an offset < 4k,
 * free the object in question.
 */

I like the touch with the config option in the last patch!

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel




> To: Christoph Lameter <cl@linux.com>
> To: David Rientjes <rientjes@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Cc: linux-mm@kvack.org
> To: "Paul E. McKenney" <paulmck@kernel.org>
> To: Joel Fernandes <joel@joelfernandes.org>
> To: Josh Triplett <josh@joshtriplett.org>
> To: Boqun Feng <boqun.feng@gmail.com>
> To: Uladzislau Rezki <urezki@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Cc: rcu@vger.kernel.org
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> Vlastimil Babka (4):
>       slab, rcu: move TINY_RCU variant of kvfree_rcu() to SLAB
>       rcu: remove trace_rcu_kvfree_callback
>       rcu, slab: use a regular callback function for kvfree_rcu
>       slab: don't batch kvfree_rcu() with SLUB_TINY
> 
>  include/linux/rcupdate.h   | 29 ++++++++++++++---------------
>  include/linux/rcutiny.h    | 36 ------------------------------------
>  include/linux/rcutree.h    |  3 ---
>  include/linux/slab.h       | 14 ++++++++++++++
>  include/trace/events/rcu.h | 34 ----------------------------------
>  kernel/rcu/tiny.c          | 24 ------------------------
>  kernel/rcu/tree.c          |  9 ++-------
>  mm/Kconfig                 |  4 ++++
>  mm/slab.h                  |  2 ++
>  mm/slab_common.c           | 32 +++++++++++++++++++++++++-------
>  mm/slub.c                  | 42 ++++++++++++++++++++++++++++++++++++++++++
>  11 files changed, 103 insertions(+), 126 deletions(-)
> ---
> base-commit: e492fac3657b60b8dd78a6e8ca26d1d14706c7b3
> change-id: 20250123-slub-tiny-kfree_rcu-bd65bfe222f2
> 
> Best regards,
> -- 
> Vlastimil Babka <vbabka@suse.cz>
>